Zinc oxide powder has been formerly frequently used in cosmetic preparations, etc. as an extender pigment or for cutting light in the UV-A region. However, since the zinc oxide powder absorbs the ultraviolet light by cleaving zinc-oxide bonds, it has a problem in that the amount of radicals generated is large among powders used in the cosmetics. For this reason, it is troublesome in compounding zinc oxide powder that the compounding amount of the zinc oxide powder and other compounded ingredients coexistent with the powder must be severely selected due to the photocatalytic activity of the zinc oxide powder. In order to suppress the photocatalytic activity of the zinc oxide powder, its surface needs to be coated with a material having a low activity, and particularly is preferably coated with an inorganic oxide. For example, there are conventional inorganic oxide-coating methods as follows. 1 As proposed in JP-A-3 183,620, etc., silicon oxide or the like is precipitated and coated upon the surface of the zinc oxide fine powder by controlling the pH in a wet state so that the solid acidic catalytic activity of the zinc oxide fine powder maybe suppressed and the coefficient of dynamic friction may be reduced. 2 As proposed in JP-A 63- 139,015, particularly in order to reduce the aggregation of a powder, a vapor of a cyclic silicone compound, or that of a straight-chain silicone compound having a low molecular weight is brought into contact with active sites of the powder, the silicone compound is polymerized by the active sites of the powder, and then the resulting polymer is fired. 3 As proposed in JP-A 8-59,238, in order to suppress the photocatalytic activity of the zinc oxide powder and improve its dispersibility, the surface of the zinc oxide powder is treated with an aluminum chelate compound and a silane compound, and the thus treated zinc oxide is coated with aluminum oxide and silicon oxide after being fired at 500.degree. C. 4 As proposed in JP-A 6-80,421, in order to obtain scaly zinc oxide flake powder having an excellent ultraviolet light shielding effect and mechanical strength, the scaly zinc oxide is precipitated in a wet state under coexistence with aluminum sulfate or the like, and then the resulting precipitant is fired at 850 to about 1,000.degree. C.
In the above methods 1 and 4, zinc oxide is dissolved in a weak acid or a weak alkali because zinc oxide is an amphoteric substance, so that zinc ions are re-precipitated upon the surface of the powder during the controlling of the pH even after a treatment. Consequently, there are problems in that the photocatalytic activity cannot be sufficiently suppressed or the powder is strongly aggregated. On the other hand, according to the above method 2, the silicon compound must be once fed, in the form of a vapor, to the powder, so that the method is troublesome and time-consuming, the amount of the treating silicone compound depends upon the catalytic activity of the powder to disable the free control of the treating amount, and the feel of the resultant powder becomes harder. According to the above method 3, since the amount of the metal is small for the large molecular weight of the aluminum chelate compound and the large molecular weight of the silane compound, the reaction needs to be assuredly effected during the coating treatment, while a byproduct needs to be removed. Further, when the zinc oxide is simply coated with the aluminum chelate compound or the silane compound and the resultant is fired, the fired coating oxide film becomes porous, so that the photocatalytic activity cannot be sufficiently suppressed.
In view of the above-mentioned problems, the present inventors strenuously examined the coating condition, the heating temperature condition, etc. for the zinc oxide powder, and consequently discovered that by surface-coating powdery zinc oxide with at least one silicone compound selected from an organo polysiloxane and a silicone resin in a non-gaseous phase, and firing the resulting powder at a temperature of 600 to 950.degree. C., the photocatalytic activity of the zinc oxide powder can be more easily suppressed, and the modified powder having an excellent feel can be more easily obtained.
Further, the inventors discovered that a cosmetic preparation containing this activity-suppressed zinc oxide powder is excellent in product stability, feel, sebum resistance, and protection against ultraviolet light.